The present invention relates to a method of correcting a position of a cutting-edge of a tool of a numerically controlled machine tool and a device for correcting the position thereof.
A conventional NC machine tool is disclosed, for example, in Japanese Patent Laid Open No. 60-146653. There is shown a numerically controlled lathe as a similler numerically controlled machine tool in FIG. 1. In the figure, reference numeral 100 designates the numerically controlled (NC) lathe which is mainly constituted by a main spindle table 101 and a tool table 200 movably mounted on a bed opposite to the table 101 in respective directions of arrows X and Z. The main spindle table 101 supports a rotatable main spindle 102 driven by a not-shown driving unit. Attached on the leading end of the main spindle 102 is a chucking unit 103 which serves to grasp a work W to be processed by the numerically controlled lathe 100. The main spindle table 101 further includes an arm 104 projecting laterally. The arm 104 is provided at the leading end thereof with a sensor S for detecting a position of a cutting edge of a tool.
The tool table 200 is supported on a not-shown table which is movable in the direction of Z, so as to move in the direction of X by the aid of a ball screw 201 which is mounted on the not-shown table to extend in the direction of X. On a side wall of the tool table 200, a turret 202 is provided with a tool holder 203 equipped with a tool T for machining the work W.
In operation of the so-constructed NC lathe 100, when the main spindle 102 is rotated by the driving unit, the work W grasped by the chucking unit 103 is rotated at a predetermined speed and then, the work W is machined by the tool T mounted on the tool table 200 moving to the directions of Z and X. In case of applying a desired processing on the work W by the NC lathe 100, the processing is carried out on a basis of a numerical control (NC) processing program installed in a numerical control (NC) unit 300 arranged in the vicinity of the NC lathe 100.
Referring to FIG. 2, there is shown a central processing unit (CPU) 301 of the NC unit 300, to which a keyboard 302 for inputting necessary data and a CRT 303 as a display unit for displaying the data, tables, figures or the like are connected. Further connected to the CPU 301 are a means 304 for correcting a position of tool and a memory section 305 in which a basic processing program is installed for producing the NC processing program in a fixed cycle. During machining, in case of correcting a position of the cutting-edge by reason of the exchange of tool attached, the wear of the cutting edge or the like, the correction is carried out by the correcting means 304 which adjusts a distance LO (FIG. 1) between an axis of the main spindle 102 and the position of cutting edge of the tool on the basic processing program.
In such a case, this correcting means 304 operates so that a constant machining dimension is always attained for the work, correcting the changes in position of cutting edge of the tool caused by the exchange of tool attached, the wear of the cutting edge or the like. In detail, the means 304 memorizes the difference between a displacement L2 shown in FIG. 1 and a predetermined standard dimension, as correction values and then executes to correct the position of the cutting edge on the basic processing program. Note, the displacement L2 corresponds to a movement amount of the tool table 200 necessary to bring the cutting-edge of the tool T into contact with the sensor S, which of course, contains a cutting-edge position correction value on memory of the means 304 at that time. The distance between the axis of the main spindle 102 and the cutting-edge of the tool T is equal to a value obtained by adding a distance L1 between the axis of the main spindle 102 and the sensor S to the displacement L2 of the tool table 200.
In this way, in the conventional NC lathe 100, a standard dimension to be determined in correcting the position of tool is a fixed constant value. Therefore, when the change in distance L1 is caused by the "thermal" displacement of the sensor S attributed to the changes in environmental temperature, heat generated from the machine itself, cutting heat or the like, this change .DELTA.L1 results in an error in measuring the displacement L2 of the tool table 200. Consequently, the error causes a correction error in position of the cutting-edge, so that it is impossible to process the work with appropriate dimensions.